Naturally occurring fungitoxicants have received a considerable amount of attention in recent years because many of them described to date are mostly biodegradable and are devoid of side effects to humans when compared with commercially available synthetic fungitoxicants. Recognition of the deleterious affect of the synthetic fungitoxicants, over a long period of time, on the health of individuals has lead biomedical scientists to actively seek information on alternate compounds with minimal adverse effect on human health. The essential oils and their components are a group of naturally occurring systems and compounds that have been investigated for their fungitoxic activity against fungi which cause dermatomycosis in animals and fungal deterioration of foods. However, the studies have primarily been limited to screening essential oils and their components against common storage and animal pathogenic fungi. The general objective of this research investigation is to determine the primary mode of action of cinnamaldehyde, eugenol, and thymol, three essential oil components isolated from cinnamon, clove, and thyme oil. During the course of the investigation, three specific objectives for the proposed study include: (1) to study the effect of cinnamaldehyde, eugenol, and thymol on branching in Aspergillus, Mucor, and Rhizopus species, (2) to study the mechanisms of fungal growth-inhibitory actions of cinnamaldehyde, eugenol, and thymol in terms of energy of molecular orbitals, and (3) to study the effect on cinnamaldehyde, eugenol, and thymol and the lipid component of fungal cell membrane. Procedures to be employed in accomplishing the specific objectives outlined in this proposed study include: the poison food technique, thin-layer chromatography, high performance liquid chromatography, gas chromatography mass spectrometric method, molecular photochemistry, visible-ultraviolet spectrophotometric methods, Lowery, orcinol, diphenylamine, amino acid and nucleotide qualitative and quantitative methods.